A decade ago I bought a B44 with oil leaks and began a research journey. After years of dead-ends and breakthroughs; I patented a new way to breathe four-stroke, air-cooled engines. It’s satisfying to help riders cut oil leaks in vintage, classic and contemporary marques. With Rickard’s fine idea of Tech Write-Ups, it’s time to turn this research back onto BSA unit singles.
For further reading on crankcase breathing, browse the 76+ research, technical and installation articles at http://bunnbreather.bigblog.com.au 100+ riders use this daily. It’s a unique resource on the subject. Because so little classic research was published, a lot of fallacies are on the net. I take an evidence-based approach and avoid them.
This article covers the three popular late unit single models i.e. B25/T25, B44 and B50. All the bikes are SS models. Like most marques, they respond to an installation with two breather valves.
1.0 1971 B25SS/T25SS-
This model has the OEM shaft breather left in situ. It can be blocked off. At the time of this pic, the bike had a 2008 Kit with matching red air filter visible in front of the electrics box. The Inlet breather drops to the timing plug drilling via a MIG welder diffuser union. The Exhaust breather exits the inlet rocker-box hatch and drops down behind the gearbox. This bike has since been updated to road-test the 2010 breather. The bike is completely oil-tight, with bags of power.
2.0 1970 B44SS- This is the left side bike below, also with the OEM breather in situ.
This was the first test-bed and a word about its history is timely.
It leaked badly from a rear barrel stud, the barrel gasket, the rocker-boxes (where over-pressure had blown out the gasket) and other places. Following solutions promoted by Dave Smith and other BSA singles experts, I installed an open rocker-box line, then a series of brake valves, followed by various PCV valves. Still I had oil leaks. No one had better answers. Worse, no one had could explain what was going on inside the engine. The bike had been restored and was otherwise in sound order. Why strip an otherwise sound engine?
As experts couldn’t help, I looked for answers in the literature. My library grew to 500 books, from Judge, Nicholson, Irving, Lupton, Bell, Robinson and Buzzelli et al. I borrowed another 500. Out of this, the coverage of crankcase breathing amounted to a few pages, under 0.0005%. Most books don’t even index it. What was written was superficial. The exceptions stand out…. sublime half pages from Williams and Lupton, two paras from Irving and Bell’s 2006 edition.
The only way forward was research. Classic designers didn’t have answers as they didn’t ask the right questions. I tested the B44 at first with a plastic bag and stopwatch (sic), then flow indicators, gauges and sensors. Now I use a set of gauges and sensors, and run a battery of tests on every bike. Along the way I consulted engineers and classic riders to keep on track. Over the decade I progressed. There’s a way to go. The B44 runs a 2009 Kit and is now dry, save that busted rocker gasket. What they say about mechanics bikes is true!
The B44 has the Inlet breather dropping from a red air filter (below left gusset) down to the timing plug union. The Exhaust breather exits the inlet rocker hatch and drops behind the gearbox, terminating in a ‘draft tube’ projecting into the airstream. This bike has also since been updated to road-test the 2010 breather.
3.0 1971 B50SS- This bike (on right in pic) retains the OEM breathers, plus the 2010 breather. This starts inside the gusset ( to stay dry) and follows the frame down to timing union. In-line valves and filter are nigh invisible, pre-assembled and quick to install. The Exhaust joins the OEM chaincase union and ends behind the gearbox with a draft tube. This bike is restored so hasn’t done many miles. It’s oiltight and running typical crankcase pressures like the others.
4.0 Discussion- Crankcase breathing has been overlooked since vintage days, but no longer. It’s now a ‘Top Three’ R&D priority for major racing teams. (The others being Friction and Cylinder Contamination) The aim of course is to cut power losses.
Our objectives for classic crankcase breathing R&D include pressure control, moisture purging and power loss reduction in that order. For the USA it’s the other way round! Moisture purging is increasingly important with ethanol fuels (see Blog 76/), where the Bunn Kit design stands alone.
We’ve a lot of experimental evidence backed by sound theory and engineering support. We now work with a leading engineering school, as more equipment is needed for ongoing research. The old forum debates re. open vs closed breathing, valve types etc. showed how little we really knew then, but spurred us on to find out. There’s still a lot of research to do.
